3.55 CFW are generally more stable, with less brick risks, can run the same games as 3.55+ CFWs and has better homebrew compatibility. I know this isn't what you asked, but It's perhaps something you should consider, It's not worth going on 3.55+ CFWs such as Rogero's 4.21 unless you want just the OFW features, such as the Vita support It has.
Btw, a friend went from 3.41.3 Rebug CFW to 4.21 CFW. In theory, there is no difference between going from 3.15 OFW to 4.21 CFW and 3.55 OFW to 4.21 CFW, there is only point in doing It if you want to install qa flag.
Btw, you're using PS3 fat right? Rogero 4.21 v1.09 is giving quite a few bricks on those. 4.30 seems to be safer, but I'd still recomend 3.55 CFW.